Md Roysul Islam

Molecular divergence estimates suggest that current taxonomic diversity and ecology of baleen whales (Mysticeti) evolved during the Late Miocene–Early Pliocene, 10.0-3.5 million years ago (Ma): a time interval marked by significant oceanic restructuring and global climatic deterioration. Although Mio-Pliocene boreal fossils suggest higher generic richness and greater ecological disparity than the present day, there is limited evidence on Mio-Pliocene mysticetes from austral seas. Late Miocene–Early Pliocene sediments from Beaumaris and Hamilton in Victoria have produced >100 diagnostic periotic and tympanic ear bones, representing the following genera: Eubalaena from Balaenidae (right whales); Balaenoptera and Plesiobalaenoptera from Balaenopteridae (rorquals); Caperea from Neobalaenidae (pygmy right whale); and Herpetocetus from the extinct family Cetotheriidae. The records of the extinct genera Plesiobalaenoptera and Herpetocetus represent the first records of these taxa. The current mysticete fauna of the southwest Pacific includes four genera: Eubalaena, Balaenoptera, Megaptera and Caperea. This compares with five genera documented in this study from the Late Miocene–Early Pliocene of Victoria. These results indicate greater generic richness during the Mio-Pliocene compared to the present day, and a maximum age of < 5 Ma for the origin of modern mysticete diversity and biogeography in the southwest Pacific.

The large to huge baleen whales (Mysticeti) are significant consumers and distributors of nutrients in marine ecosystems, especially those of the Southern Ocean. Molecular divergence estimates imply that these ecological interactions and broad taxonomic diversity must have evolved during the Late Miocene–Early Pliocene (10–3 Ma), which is characterized by successive climatic shifts and reorganization of currents and productivity. Northern hemisphere fossil data suggest a general pattern of higher generic diversity and ecological disparity than present during this interval. However, fossil evidence on mysticete evolution in the Southern Ocean and its margins during this pivotal time is largely undocumented, hampering comparison between boreal and austral patterns. The Upper Miocene–Lower Pliocene Grange Burn Formation and Black Rock Sandstone of Victoria have yielded a moderate sample (~70) of mysticete ear bones (periotics and tympanic bullae) that may inform this question. Preliminary study shows that the fossil assemblages includes representatives of at least three living families: Balaenidae (right whales), Balaenopteridae (rorquals, e.g. humpback whales) and Neobalaenidae (pygmy right whale). This implies that family-level diversity during the Mio-Pliocene was at least equivalent to that of the Southern Ocean today. It remains unclear whether the extinct (and widely distributed) Cetotheriidae are present in the Victorian fossil assemblages. Below the family level, Balaenidae and perhaps Balaenopteridae seem to be represented by some extinct genera. We anticipate that mysticete ecological diversity was broadly equivalent in penecontemporaneous Southern Ocean and northern hemisphere oceans during the Late Miocene–Early Pliocene.